Campbell Scientific CR7 Measurement and Control System - Manual (Page 171)

D-1
APPENDIX D. CALIBRATION PROCEDURES
The CR7 requires very little maintenance or calibration. Measurements are made in such a way that
small errors in the calibration are automatically removed. Over time, shifts in the calibration are
possible, however. Measurements can be made to determine whether the accuracy of the CR7 is within
the specifications given in Section I.3. If needed, the calibration procedures described in this section can
be performed by an experienced technician having the suggested equipment.
NOTE: The precision of the CR7 exceeds that of most standard electronic equipment. These
procedures require that the test equipment have a precision equal to or better than the CR7.
The following procedures are for calibrating the voltage reference and the clock. Other factors such as
range ratios, DAC non-linearity, and offset in either the switched excitation or the CAO voltage require
that the CR7 be returned to the factory for repair. Please call the factory to obtain authorization before
sending in the unit.
D.1 VOLTAGE REFERENCE
CALIBRATION PROCEDURE
The following procedure assumes that the CR7
being calibrated has an analog input card
configured as analog input card #1 and an
excitation card configured as excitation card #1.
Adjustments would need to be made in the
programming example if the cards are
numbered differently.
SUGGESTED INSTRUMENTS:
Five and one half digit digital volt meter (DVM)
with 10 microvolt resolution. The accuracy of
the DVM needs to be equal to that of the CR7
which is ±1 mV at 5 VDC.
PROCEDURE:
1. With a small, flat screw driver, pry off the
three silver caps on the top of the Analog
Interface Card in the I/O module (see
Figure D.2-1).
2. Monitor the Digital to Analog Converter
(DAC) output by connecting the positive
lead of the DVM to the DAC OUTPUT
TEST JUMPER. Connect the negative lead
of the DVM to a ground on the analog input
card. Set the DVM to read on its most
sensitive DC Volt scale.
3. Set the DAC to output 0.000 volts DC, by
programming the CR7 as follows:
* 1 Table 1 Programs
01: 1 Sec. Execution Interval
01: P22 Excitation with Delay
01: 1 EX Card
02: 1 EX Chan
03: 900 Delay w/EX (units=.01sec)
04: 0000 Delay after EX (units=.01sec)
05: 0.0000 mV Excitation CHANGE
AS INSTRUCTED
02: P End Table 1
* 0 Compiles the instructions
4. Adjust the DAC OFFSET
POTENTIOMETER for a DVM reading of
0.0000 V (±0.0001 V).
5. Change the CR7's program given in step
three so that parameter 5 reads 4000 to
change the DAC output to 4 V. Set up the
DVM to read 4 VDC.
6. Adjust the VOLTAGE REFERENCE GAIN
POTENTIOMETER until a DVM reading of
4.0000 V (±0.001 V) is obtained.
7. Change the CR7s program given in step 3
so that parameter 5 reads -4000 to change
the DAC output to -4V.
8. A DVM reading of -4.000 V (±0.001 V)
verifies the DAC linearity.
9. Lock both potentiometers into position with
a dab of finger nail polish.